1. Field of the Invention
The present invention is directed to photoluminescent phosphorescent formulations, comprising an effective amount of photoluminescent phosphorescent materials, that exhibit high luminous intensity and persistence. The photoluminescent phosphorescent formulations may further comprise photoluminescent fluorescent materials, wherein said photoluminescent fluorescent materials increase the luminous intensity and persistence. These photoluminescent fluorescent formulations may further comprise photostabilizers to retard the photolytic degradation of said photoluminescent materials.
The present invention is also directed to photoluminescent objects comprising at least one photoluminescent formulation and a preformed article. Further disclosed are methods for creating photoluminescent objects comprising applying at least one photoluminescent formulation onto a preformed article.
2. Description of Related Art
Consumers have a continuing desire to receive added informational features and benefits from the products that they purchase. Such information and features can comprise safety information indicators, environmental information indicators, shelf-life information indicators, authentication and tamper indicators, fashion accessory benefits and/or fun and entertainment benefits. Color-change technology triggered by specific environmental events can form the foundation for creating these informational indicators or benefits. It is important to note that for the utilization of color-change as informational indicators or providing additional benefits to be effective, such color change needs to be visually striking and for outdoor usage environmentally robust.
“Envirochromic Materials” and “Envirochromic Layers” are those, when triggered by a specific environmental change or occurrence, that can change their visible color which can result from either onset or change in electromagnetic radiation emission, and/or change in the absorption reflection, and/or scattering of electromagnetic radiation. These environmental “triggers” include change in temperature, change in electromagnetic radiation, change in chemical environment, an electrical stimulus, etc.
Since color change can occur from a multiplicity of triggers, the word “chromic,” as used herein, signifies a color change occurring from change in reflection, absorption, or scattering of electromagnetic radiation. “Chromic,” as used herein, does not signify a color change occurring from change in emission. Thus, for example: photochromism signifies color change triggered by electromagnetic radiation; thermochromism signifies color change triggered by change in temperature; electrochromism signifies change in color occurring due to gain or loss of electrons; solvatochromism signifies color change resulting from change in solvent polarity; halochromism signifies color change caused by a change in pH; ionochromism signifies color change caused by ions; tribochromism signifies change in color caused by change in mechanical friction; and piezochromism signifies change in color caused by change in mechanical pressure.
As can be appreciated, color change can also result from luminescent emissions. For such a case, and being consistent with the definition above, “luminescent” or “luminous,” as used herein, signifies color change resulting from emissions.
The term “luminescence” is defined as the emission of electromagnetic radiation from any substance. Luminescence occurs from electronically-excited states. As seen in FIG. 1, absorption of ultraviolet radiation by a molecule excites it from a vibrational level in the electronic ground state to one of the many vibrational levels in the electronic excited states. The electronic states of most organic molecules can be divided into singlet states and triplet states.
As used herein, “singlet state” refers to when all electrons in the molecule are spin-paired. As used herein, “triplet state” refers to when one set of electron spins is unpaired. The excited state is usually the first excited state. A molecule in a high vibrational level of the excited state will quickly fall to the lowest vibrational level of this state by losing energy to other molecules through collision. The molecule will also partition the excess energy to other possible modes of vibration and rotation.
“Luminescent materials” are those which emit electromagnetic radiation. Characterizing luminescent materials requires consideration of: (1) the excitation source, (2) the nature of the emission, and (3) whether or not additional stimulation is required to cause emission.
With regard to the excitation source, luminescent materials excited by electromagnetic radiation are referred to herein as “photoluminescent.” Luminescent materials excited by electrical energy are referred to herein as “electroluminescent.”
Luminescent materials excited by a chemical reaction are referred to herein as “chemiluminescent.”
With regard to the nature of the emission, this may be either fluorescence or phosphorescence. A “fluorescent” material stores electromagnetic radiation and releases it rapidly, in about 10−12 seconds or less. Contrarily, a “phosphorescent” material stores electromagnetic radiation and releases it gradually, in about le seconds or more.
Processes that occur between the absorption and emission of electromagnetic radiation are usually illustrated using a Jablonski Diagram, such as the one found in FIG. 2. Ground, first, and second electronic states are depicted in FIG. 2 by S0, S1, and S2, respectively. At each electronic energy level, the fluorophores can exist in a number of vibrational energy levels, denoted by 0, 1, 2, etc. Transitions between states are depicted in FIG. 2 as vertical lines to illustrate the instantaneous nature of electromagnetic radiation absorption.
“Fluorescence” occurs when a molecule returns, by emission of a photon, from the excited singlet state to the electronic ground state. If the photon emission occurs from S1 to S0, it is characterized as fluorescence.
“Phosphorescence” occurs when a molecule goes from the ground state to a metastable state such as T1, a triplet state, and then the metastable state slowly decays back to the ground state S0, via photon emission. Hence, if the emission occurs between T1 to S0, it is characterized as phosphorescence.
With regard to whether or not additional stimulation is required to cause emission, as used herein, the need for “additional stimulation” is based upon the predominant behavior of the material at about room temperature, i e., at about 10° C. to about 35° C. Thus, in cases where electromagnetic radiation is used to stimulate emission at room temperature, such materials are referred to as “photoluminescent photo-stimulable.” In cases where electrical energy is used to stimulate emission at room temperature, such materials are referred to as “photoluminescent electrically-stimulable.” In cases where thermal energy is used to stimulate emission at room temperature, such materials are referred to as “photoluminescent thermally-stimulable.”
The instant invention applies and uses photoluminescent phosphorescent materials to cause nighttime emissions, which may additionally comprise photoluminescent fluorescent materials to enhance the intensity and persistence of the nighttime emission and/or the color of the daytime and nighttime emissions. In the past, metal sulfide pigments were used in an attempt to arrive at photoluminescent phosphorescent materials. See, e.g., U.S. Pat. No. 6,207,077 to Burnell Jones. A common such metal sulfide pigment is a zinc sulfide structure whereby the zinc is substituted and activation occurs via various elemental activators, coactivators, or compensators. Common activators include copper, aluminum, silver, gold, manganese, gallium, indium, scandium, lead, cerium, terbium, europium, gadolinium, samarium, praseodymium, and other rare-earth elements and halogens. These activators are believed to enter the crystal lattice of the host material and are responsible for imparting the luminescent properties to the material.
Examples of sulfide phosphorescent phosphors include CaS:Bi, which emits violet blue light; CaStS:Bi, which emits blue light; ZnS:Cu, which emits green light; and ZnCdS:Cu, which emits yellow or orange light. However, these sulfide phosphorescent phosphors are chemically-unstable and, as a result, exhibit photolytic instability
An extensively-used sulfide phosphorescent phosphor is zinc sulfide, ZnS:Cu. See, e.g., U.S. Pat. No. 3,595,804 to Martin. However, zinc sulfide decomposes due to irradiation by ultraviolet radiation in the presence of moisture. This decomposition reduces and/or blackens the luminance, making the use of zinc sulfide difficult in environments containing ultraviolet radiation and/or moisture. As a result, zinc sulfide is used most-commonly in controlled environments, such as for clock, watch, and instrument dials, and for indoor uses. Relatively recently, see, e.g., U.S. Pat. No. 5,424,006 to Murayama, metal aluminate photoluminescent pigments, particularly alkaline earth aluminate oxides having the formula MAl2O4, where M is a metal or mixture of metals, have been developed. Examples of such alkaline aluminate oxides include strontium aluminum oxide, SrAl2O4, calcium aluminum oxide, CaAl2O4, barium aluminum oxide, BaAl2O4, and mixtures thereof. These aluminate phosphors, with or without added magnesium, may be further activated with other metals and rare-earth elements.
These aluminate photoluminescent pigments exhibit afterglow characteristics that last much longer in duration than do those of metal sulfide pigments. These aluminate photoluminescent pigments also exhibit strong photolytic stability and are chemically more stable than the metal sulfide pigments. For example, strontium aluminum oxide, SrAl2O4, such as that disclosed in U.S. Pat. No. 5,698,301 to Yonetani, exhibits luminance that is about five- to ten-times that of zinc sulfide phosphoresecent phosphor, ZnS:Cu, and exhibits a smaller decay rate. Strontium aluminum oxide also exhibits an emission spectrum having a peak wavelength of 520 nanometers (“nm”), which is near the spectrum of peak human visibility, and exhibits a broad excitation spectrum with high excitation efficiency to ultraviolet electromagnetic radiation in the short wavelength region.
However, alkaline earth phosphors, such as strontium aluminum oxide, exhibit the disadvantage of requiring more excitation time to attain saturation luminance than do metal sulfide pigments, such as zinc sulfide phosphoresecent phosphor. In addition, alkaline earth phosphors have the disadvantage of moisture sensitivity. On the other hand, sulfide-based phosphors degrade photolytically.
It can be appreciated that for optimal luminescent performance, specific photoluminescent phosphorescent materials and mixtures of such materials need to be adapted for use in varying conditions, be it excitation conditions or environmental considerations. Water-resistant formulations suitable for protecting the photoluminescent phosphorescent particles and formulations that minimize photolytic degradation, particularly where metal sulfides are utilized, are sought-after. Beyond the selection of the photoluminescent phosphorescent materials and/or any additional photoluminescent fluorescent materials used to enhance their performance, it should be noted that the luminous intensity and/or persistence from a photoluminescent formulation is greatly affected by both the way in which the photoluminescent phosphorescent material is distributed and the additives used, as well as the manner in which that formulation is applied. As noted above, for these materials to serve as color-change indicators or to provide added information and/or benefits to consumers, the color change needs to be visually striking to be effective.
The improper selection and use of formulation materials, such as resins, dispersants, wetting agents, thickeners, and the like can diminish the luminous intensity emanating from the formulation. This can occur, for example, due to agglomeration or settling of photoluminescent phosphorescent particles, either during handling of the formulated materials or after application of the formulated materials. The reduction in luminous intensity and/or persistence can result from both incomplete excitations and/or due to scattering of emitted radiation. The scattering of photoluminescent emissions can be either due to agglomeration of photoluminescent phosphorescent material or as a consequence of electromagnetic radiation scattering by of one or more of the additives selected to stabilize the photoluminescent phosphorescent pigment dispersion. The net result will be lower luminous intensity and persistence.
By and large, the current practice in commercially-available materials is to cite the luminous intensity and persistence of the underlying photoluminescent phosphorescent powder, rather than that of the resulting photoluminescent object. It can be recognized that for commercial success, the important parameter is not the photoluminescent intensity and persistence of the underlying powder, but that of the resulting photoluminescent object. There is a need, therefore, to not only develop photoluminescent phosphorescent powder materials of high performance but also develop photoluminescent formulations that result in photoluminescent emissions of high intensity and persistence.
Articles having inadequate reflection to the emitted electromagnetic radiation, either because of surface roughness or because of their color resulting from materials that are absorptive of photoluminescent phosphorescent emissions, can also result in degradation of luminous intensity and persistence even when high-performance formulations are applied to such articles to create photoluminescent objects. Further, outdoor usage of photoluminescent objects also necessitates, beyond good adhesion to substrates, mechanical robustness such as scratch resistance, etc. Specific requirements are dictated by the particular application, for which the use of a protective layer can also be highly beneficial. It can therefore be seen that beyond the need to develop photoluminescent formulations of high performance, there is a need for a multi-layer system construction for applying these formulations to articles to create photoluminescent objects of high intensity and persistence of nighttime emissions.
The use of colorants in the form of pigments that are absorptive of visible electromagnetic radiation to impart daylight color to photoluminescent formulations even when they are not absorptive of phosphorescent emissions can result in degradation of photoluminescent intensity and persistence by virtue of either scattering of photoluminescent phosphorescent emissions or by inadequate charging of photoluminescent phosphorescent materials. The latter phenomenon can result if the particle size of the absorptive colorants is small enough. Hence, while absorptive colorants can be used to alter the daytime appearance of photoluminescent objects, such usage will result in a lowering of luminous intensity and persistence. This is why a majority of daylight-colored formulations are rated for low intensity and persistence. Further, such usage also precludes the achievement of daytime colors and nighttime emissions being in the same family of colors.
U.S. Pat. No. 6,359,048 to Duynhoven discloses formulations of photoluminescent phosphorescent materials utilizing alkyd resins and modified castor oil rheology modifiers. This formulation requires using a secondary pigment particle, which, due to scattering of electromagnetic radiation, results in lower transmissivity of photoluminescent phosphorescent emissions, and thus lower perceived intensity and persistence of emissions from objects deploying this formulation.
U.S. Pat. No. 6,773,628 to Kinno discloses formulations of photoluminescent phosphorescent materials comprising synthetic cellulosic resin binders and silica-based powders used as suspending fillers. The silica-based suspending fillers, by virtue of either scattering of luminescent phosphorescent emissions, or, if sufficiently small, by virtue of scattering of charging radiation, will result in a lowering of perceived intensity and persistence of luminescent objects deploying this formulation.
Published U.S. Patent Application No. 2003/0222247 to Putman discloses the use of absorptive pigments as colorants for imparting daytime color. Again, as discussed above, on account of scattering of photoluminescent phosphorescent emissions, the intensity and persistence of nighttime emissions from photoluminescent objects deploying this formulation will be lowered.
U.S. Pat. No. 3,873,390 to Cornell discloses a method of making single-layer photoluminescent phosphorescent or fluorescent films utilizing silica gel, which scatters electromagnetic radiation, to disperse the phosphorescent or fluorescent pigments. While this makes the film translucent, again, as stated above, there will be a reduction in photoluminescent intensity and persistence. Moreover, since the resin matrix selected for the pigments is a hot-melt adhesive, it requires heating coating fluid to temperatures in vicinity of 2950° F. or higher. The resulting application methodology is too restrictive for many applications.
U.S. Pat. No. 4,208,300 to Gravisse discloses single-layer phosphorescent coatings which comprise “Crystalline charges” in amounts of 50% to 65% by weight of the phosphorescent layer. Use of such high amounts of crystalline fillers is indicative of a basic composition that has low transmissivity to phosphor emissions without the crystalline fillers. Uses of such high amounts of filler material will not only result in significantly lower concentrations of phosphorescent pigments, but also, since these fillers are silica-based, they will also result in a lowering of luminous efficiency.
U.S. Pat. No. 4,211,813 to Gravisse discloses the making of flexible photoluminescent articles comprising a single-layer phosphorescent coating for applications requiring high water vapor transmissivity. This requirement does not result in a degradation of the phosphor materials, since they are ZnS-based. It is now well-appreciated that the photoluminescent intensity and persistence of ZnS-based materials is significantly lower, as compared to the newer alkaline earth materials which however can be degraded by water. Hence, the need remains for construction of photoluminescent objects that have low water vapor transmission and still exhibit nighttime emissions of high intensity and persistence. U.S. Pat. No. 5,698,301 to Yonetani teaches the construction of a phosphorescent article embodying a three-layer construction, that is, a reflective layer, a photoluminescent layer, and a clear protective layer, without use of photoluminescent fluorescent materials. This invention does not require specific performance characteristics of each of the layers. With respect to the photoluminescent layer, all that is suggested is “dispersing a phosphorescent pigment in a varnish prepared by dissolving a resin in a solvent thereby preparing an ink.” Alkaline earth materials, such as strontium aluminates, are not easy to disperse and unless one achieves a construction of such a layer without photoluminescent phosphorescent particle agglomeration, there will be loss of efficiency due to incomplete charging. Also, since photoluminescent phosphorescent materials have high densities, without using specific additives, there will be settling and compaction as the film dries, resulting in a lower amount of nighttime emissions from the surface. It should also be noted that common additives for addressing these issues, e.g., silica, scatter electromagnetic radiation, causing the layer's transmissivity to photoluminescent phosphorescent emissions to be lower.
U.S. Pat. No. 5,395,673 to Hunt discloses the construction of a non-slip phosphorescent surface by applying to a ground surface epoxy resin containing compositions impregnated with phosphor pigment of the zinc sulfide type. The focus of this invention is on the creation of a hard surface with photoluminescent phosphorescent materials incorporated therein, and not on methodologies to maximize intensity and persistence of nighttime emissions.
U.S. Pat. No. 5,692,895 to Franzin Nia discloses the rudimentary concept of a photoluminescent phosphorescent orthodontic appliance utilizing the older, zinc sulfide-type phosphors. The phosphorescent pigment can be deposited onto the exposed bracket surfaces utilizing methods such as glazing, ion beam implantation, plasma coating, and the like. However, since the appliances are based on the older sulfide-type photoluminescent phosphorescent materials, the resulting intensity and persistence of the emissions will be significantly lower and, further, the materials will be subject to rapid photolytic degradation.
U.S. Pat. No. 6,207,077 to Burnell-Jones discloses the application of photoluminescent phosphorescent coatings to fiber optic articles using a curable layer construction, as well as a variety of fillers. The fillers include suspending fillers, such as silica, for preventing settling of phosphor particles, tailoring viscosity, etc. The heavy loading of filler materials, in the neighborhood of 30%, negatively impacts the amount of photoluminescent material present, thus requiring much thicker photoluminescent layers. In addition, due to scattering of electromagnetic radiation from the quantity and type of filler materials deployed, there will be a reduction in the intensity and persistence of nighttime emissions from the objects deploying this formulation.
U.S. Pat. No. 6,508,732 to Romberger discloses the construction of a tennis ball that includes an outer fabric cover that contains a photoluminescent phosphorescent component. The object of this invention is a luminescent tennis ball and not on methodologies to maximize intensity and persistence of nighttime emissions.
Accordingly, in view of the above, there remains a need for photoluminescent phosphorescent material formulations, photoluminescent phosphorescent objects, and methods for creating such objects, wherein the formulations and objects not only exhibit high intensity and persistence, but also which can be created in a variety of daytime and nighttime colors, also with high luminous intensity and persistence, and additionally including creation of photoluminescent objects wherein the daytime and nighttime colors are in the same family. The photoluminescent objects are also created to minimize photolytic degradation, do not degrade with moisture, and are mechanically robust, particularly in outdoor applications.